1. Field of the Invention
The present invention relates to improvements in a portable measuring instrument which utilizes as a power source a group of solar batteries, in which a plurality of said solar batteries are series-connected on the same plane.
2. Description of the Prior Art
Solar batteries have previously been utilized as a power source for portable measuring instruments such as a slide caliper and a micrometer (see, for example, Japanese Patent Application No. 1340/1990 filed by the present applicant).
In general, when solar batteries are utilized as a power source for a portable measuring instrument, usually a plurality of single cells of solar batteries are series-connected in order to provide a voltage level required for powering the portable measuring instrument.
FIGS. 3A through 3D show conventional arrangements in which four solar batteries 12 formed of single cells including a cell `a`, 12a, a cell `b`, 12b, a cell `c`, 12c, and a cell `d`, 12d are series-connected. In FIG. 3A, the cell `a`, 12a, is connected to the cell `b`, 12b, the cell `b`, 12b, to the cell `c`, 12c, and the cell `c`, 12c, to the cell `d`, 12d. Thus, as a whole, the four solar batteries 12 are series-connected. A negative electrode terminal 28 is disposed at one end of the cell `a`, 12a, and a positive electrode terminal 30 at one end of the cell `d`, 12d. Connected to the positive electrode terminal 30 and the negative electrode terminal 28 is a load circuit 32, through which a current I flows. Connection and function in FIGS. 3B through 3D are the same as in the above.
In general, it is known that, when a plurality of solar batteries 12 formed of the single cells are series-connected, the magnitude of the current I flowing through the load circuit 32 is determined by the amount of current in the cell of solar battery 12 where the minimal current flows. FIGS. 6A through 6C show the above-described fact. FIGS. 6A and 6B show the relationship between the current and the voltage in the solar batteries when the cell `a` and cell `b`, in their single states, have maximal electromotive voltages of Va, Vb and maximal currents Ia, Ib, respectively. When the cell `a` and the cell `b` are series-connected as shown in FIG. 6C, the maximal voltage is Va+Vb, while the maximal current is Ia. Part of the current Ib-Ia is consumed as the heat loss by recombination of electrons and holes.
FIG. 4 shows a conventional example in which a group 8 of solar batteries series-connected as shown in FIG. 3 are mounted on a slide caliper 60 as an example of a portable measuring instrument 10. The slide caliper 60 has an electrostatic capacity type position sensor (see, for example, Japanese Patent Laid-Open No. 212711/1984 filed by the present applicant). The slide caliper 60 has a slider part 46, slide caliper 60 being slidable on a fixed stem having fixed part 44 arranged thereon with the fixed stem having electrodes, not shown, of a predetermined pitch. On slide caliper 60 is mounted electrodes, not shown, of a predetermined pitch, which are opposed to electrodes of the fixed stem, a load circuit (see FIG. 3A) for driving the electrodes and processing a measuring signal, and a group 8 of solar batteries serving as a power source for supplying current to the load circuit. A display 34 is disposed in the central portion of the group 8 of solar batters. Designated at 40 is an on/off switch for operating the power source and 42 is a ZERO switch for zero-setting an original point of measuring. Denoted at 48A is a projection to be pressed by a hand 16 or the like for moving the slider part 46, and 48B a rotating wheel for moving the slider part 46 relative to fixed part 44.
Furthermore, FIG. 5 shows a conventional example in which the group 8 of solar batteries series-connected as shown in FIGS. 3A through 3D are mounted on a micrometer 62 as an example of the portable measuring instrument 10.
When thimble 68 is rotated, spindle 66 is advanced, whereby the distance between the spindle 66 and an anvil 64 is measured.
Designated at 34 is a display for displaying the result of measuring and 8 designates the group of solar batteries.
However, there has heretofore been the problem that part of the light which irradiates the light receiving windows of the solar batteries 12 are screened by the hand 16 or other means for operating and holding the portable measuring instrument 10.
Namely, when part of the light which irradiates the windows to group 8 of solar batteries is screened by the hand 16 or the like as shown in FIGS. 4 or 5, a light screened region 50 is formed on the light receiving windows of the group 8 of solar batteries. In the arrangements of the group 8 of solar batteries as shown in FIGS. 3A through 3D, the light screened region 50 is not substantially uniformly distributed onto the respective light receiving windows and is biased only to a specific light receiving window of the solar batteries 12.
As a result, assuming that the intensities of the irradiating light are uniform at the locations of the light receiving windows, the current I of the group 8 of solar batteries, with which the screened solar battery 12 is series-connected, is governed by the current of the most screened solar battery 12, which has the least current of all the series-connected solar batteries 12. For this reason, there has heretofore been the problem that only a specific light receiving window is significantly subjected to the influence of light screening by the hand 16 or the like for operating and holding the portable measuring instrument 10, so that the current I of the group 8 of the series-connected solar batteries as a whole is greatly reduced.